1. Field of the Invention
The present invention relates to a power transmission apparatus for a vehicle, interposed between a front wheel driving shaft and a rear wheel driving shaft of a four-wheel drive vehicle which drives the front and rear wheels by the same engine, and more particularly to a power transmission apparatus using a hydraulic pump and a four-wheel drive vehicle using the power transmission apparatus.
2. Description of the Prior Art
The four-wheel drive vehicle transmitting a driving force of its engine to the front and rear wheels thereof during the vehicle's running can surely and not-wastefully transmit the driving force onto the road surface, thereby being superior not only in the running efficiency for the road surface of low friction coefficient, or a rough surface such as a gravel road, but also in the accelerating ability and stability during the high speed running even on the usual road surface in comparison with a two-wheel drive vehicle. Hence, the four-wheel drive vehicle has especially recently been spotlighted of its comfortable high speed running without being affected by road condition and a change in weather.
The simplest construction of such four-wheel drive vehicle is to rigidly couple the drive shafts of the front and rear wheels by use of, for example, a dog clutch, which is advantageous in that a driving force of an engine is effectively distributed to the front and rear wheels corresponding to ground pressure, but deflective in that, when a difference between the turning radii of the front and rear wheels during the turning causes a difference between the number of rotations of the front wheels and that of the rear wheels, the difference in the number of rotations cannot be absorbed to thereby cause the so-called tight corner braking phenomenon of dragging the inside rear wheel in slipping and have a drawback in cornering characteristic, resulting in a fear that a propeller shaft between the front and rear wheels is twisted to cause abnormal wearing at a tire.
Accordingly, the conventional four-wheel drive vehicle mainly interposes a power transmission apparatus using a differential gear to permit a rotation speed difference between the front wheel driving shaft and the rear wheel driving shaft.
The differential gear, however, has a characteristic that an output shaft subjected to larger resistance between two output shafts is rotated slower. Hence, the four-wheel drive vehicle provided with such a power transmission apparatus has a drawback that when either of the front wheel or the rear wheel is idle, almost all the driving force of engine is transmitted to the idling wheel, but not to the other drive shaft. In order to eliminate the above drawback, the power transmission apparatus must be provided with a differential locking mechanism which stops operation of the differential gear, or a differential limit mechanism using, for example, wet multiple disc clutch means, for restricting the above differential operation, when the rotation speed difference between both drive shafts exceeds a predetermined value, thereby being considerably complicated in structure.
As above-mentioned, the power transmission apparatus between the front and rear drive shafts at the four-wheel drive vehicle is preferable to have the characteristic that, when the rotation speed difference between both the driving shafts is smaller, both the driving shafts are coupled loosely as the differential gear so that the rotation speed difference is sure to be absorbed, and on the other hand, when the rotation speed difference is larger, both the drive shafts are coupled tight as the aforesaid dog clutch, so that the driving force is properly distributed to both driving shafts corresponding to ground pressure of each tire of the front and rear wheels. The power transmission apparatus realizable by simple construction of both the characteristics contrary to each other uses a hydraulic pump as disclosed in the Japanese Patent Laid-Open Gazettes No. Sho 60-104426 and No. Sho 61-249827.
The power transmission apparatus in the former has a simple construction such that a rotor rotatable together with either one of both the driving shafts is disposed in a casing rotatable together with the other and coaxially with the rotor so as to constitute a hydraulic pump and a proper throttle is provided at a discharge oil passage extending from the pump. Hence, when a rotation speed difference occurs between both the driving shafts, pressure corresponding to the rotation speed difference is created within the hydraulic pump. In this power transmission apparatus, the rotor and casing are coupled with each other by a resistance force (static pressure), the resistance force increasing or decreasing corresponding to variation in the pressure generated by the hydraulic pump, in other words, variation in the rotation speed difference between both the driving shafts, whereby both the drive shafts are coupled tight when the rotation speed difference therebetween is larger, and loosely when the same is smaller.
The power transmission apparatus in the latter gazette is provided between the discharge oil passage and a suction oil passage extending from the hydraulic pump with a by-pass oil passage having on the way a variable throttle, the variable throttle being adjusted to control a flow rate of pressure oil in the by-pass passage, thereby making variable the generated pressure under the same rotation speed of the hydraulic pump, in other words, the resistance force between the rotor and the casing. For example, when the rotation speed difference between both the driving shafts is larger, an opening ratio of the variable throttle is automatically reduced to enable a degree of coupling between both the driving shafts to be tighter, whereby the power transmission apparatus is remarkably advantageous in that the sure running is possible even on a road surface of snowy road of low friction coefficient, or on a rough road surface of a gravel road.
Now, the power transmission apparatus for the fourwheel drive vehicle, as above-mentioned, is preferable to have the characteristic to couple the front wheel drive shaft and rear wheel drive shaft as tight as possible, so that, for example, when either the front wheels or the rear wheels are idle to create a rotation speed difference between the front and the rear wheels, the rotation speed difference therebetween is reduced to reliably transmit almost all the driving force of the engine to the four wheels. In the power transmission apparatus disclosed in the Japanese Patent Laid-Open Gazette No. Sho 60-104426, however, the maximum degree of coupling between the front and rear drive shafts depends on adjusting pressure of a relief valve provided at the discharge oil passage extending from the hydraulic pump, so that when the rotation speed difference between the front and the rear wheels exceeds a predetermined value, a constant torque is transmitted regardless of quantity of the difference, that is, the flow rate, whereby a separate control system is required to distribute a torque corresponding to road condition, thereby creating the defect in that control is complicated and a manufacturing cost also becomes high.
Also, in the power transmission apparatus disclosed in the Japanese Patent Laid-Open Gazette No. Sho 61-249827, besides the problem of the relief valve, the maximum degree of coupling obtained thereby depends not on the opening ratio of the variable throttle provided at the by-pass oil passage, but on that of a stationary throttle provided in the discharge oil passage juxtaposed with the by-pass passage. Since the reduction of opening ratio of the stationary throttle is limited in processing accuracy and also the excessive reduction of opening ration creates a new problem in that clogging occurs by contamination, the rise of maximum degree of coupling is limited. Furthermore, since the by-pass oil passage is formed, there is the defect that the passage is complicated in constitution.
On the other hand, a four-wheel drive vehicle is provided with a four-wheel steering device as one means for preventing the tight corner braking phenomenon, which connects the front and rear wheels tightly and steers the front and rear wheels separately from each other. Such four-wheel drive vehicle, however, must independently be provided with a driving system for the four-wheel driving and a steering system for the four-wheel steering, whereby there is a drawback that the system is complicated, a vehicle body weight increases to increase the number of parts and the manufacturing cost becomes high.